Assessment of Critical Spans

Example of a long transmission line with several angles (line azimuths) in a complex wind field. Line rating can vary substantially on a span-by-span level. The critical span is usually located where there are parallel winds.

Limiting Spans Impact the Overall Line Ratings

At any given time, every span along a transmission line has unique set of physical characteristics and is exposed differently to local weather conditions. A conservative and safe overall dynamic line rating will consider every span along the line because the overall dynamic rating corresponds to the lowest rated value. The span where lowest rating value occurs is referred to the limiting or critical span. The limiting span is time and space varying according to variations of wind speed and direction.

Assuming prevailing wind conditions for a region, it is possible to identify the climatological critical spans by applying DLR using historical weather data sets.

Why weather-based DLR is key

Wind speed and direction are the most influential and variable parameters that effects a line's ratings. Some lines have long extents and will have several angles of orientation (line azimuths). Even assuming spatially uniform wind, long lines with a high number of line azimuths will have a higher probability of having at least one span encountering parallel winds. This span with a low angle of incidence with the wind vector (parallel wind) will significantly reduce the overall dynamic line rating of the line. This situation cannot be captured by any direct contact sensors without it becoming cost prohibitive. Therefore, it is essential and much more cost-effective to account for line orientations and complex wind fields by applying some level of wind field modeling.